1. Field of the Invention
The invention is related to the field of communication networks and, in particular, to systems and methods for synchronizing call traffic in the forward direction transmitted over backhaul links from a TPU (traffic processing unit) to base stations that are serving a mobile device in the downlink (i.e., forward direction).
2. Statement of the Problem
A typical wireless or mobile network includes a plurality of base stations that communicate via radio frequencies to mobile devices, such as cell phones. Each base station also connects to a traffic processing unit (TPU), which may be implemented in Mobile Switching Center (MSC), a Base Station Controller (BSC), or a similar switching office over a backhaul network. The mobile device transmits frames for a call (i.e., in the uplink direction) to the base station, and the base station transmits the frames, which is referred to herein as reverse call traffic, to the traffic processing unit over a backhaul link. The traffic processing unit thus receives this call traffic in the reverse direction (base station to TPU). The base station also receives call traffic for the call, which is referred to herein as forward call traffic, from the traffic processing unit over the backhaul link. The traffic processing unit thus transmits this call traffic in the forward direction (TPU to base station).
In a call involving a mobile device, the mobile device typically communicates with multiple base stations that are in range of the mobile device. If the user of the mobile device is speaking, the mobile device digitizes the voice and inserts the digitized voice into a sequence of frames. The mobile device then transmits the frames (in sequence) associated with the call to the multiple base stations in the reverse direction. Responsive to receiving the frames, each base station transmits the frames to the traffic processing unit according to a preset time interval (e.g., 20 ms) over its associated backhaul link. The traffic processing unit then processes the frames from the call to perform frame selection. For instance, for the first frame in the sequence, the traffic processing unit selects which of the frames received from the base stations is best, and the selected frame is used as the first frame in the sequence. For the second frame in the sequence, the traffic processing unit selects which of the frames received from the base stations is best, and the selected frame is used as the second frame in the sequence. This frame selection process is repeated for each frame of the call. The traffic processing unit then forwards the selected frames to the core network.
In the forward direction, the traffic processing unit receives frames for the call from the core network. These frames represent the voice of the other party to the call in full-duplex communications. The traffic processing unit then broadcasts each frame of the call to the base stations over their associated backhaul links as forward call traffic. The traffic processing unit broadcasts the frames to the base stations according to a preset time interval (e.g., 20 ms). Responsive to receiving the frames for the call from the traffic processing unit, each base station transmits the frames to the mobile device. The base stations are synchronized so that they transmit the same frame of the sequence at the same time. The mobile device then receives the frames and converts the digitized voice in the frames to analog voice signals, and provides the voice signals to the user so that the user may hear the spoken voice of the other party to the call.
The base stations serving the call may be communicating with the traffic processing unit over different types of backhaul links. A common type of backhaul link is a point-to-point T1/E1 link. However, other types of switched backhaul facilities may be used, such as an IP network. Also, different transport protocols may be used for transmission of frames, such as ATM, Frame relay, etc. Because different base stations serving the same call may use different backhaul links and/or transport protocols, there may be a differential delay between call traffic as transported over the backhaul links.
For reverse call traffic, the traffic processing unit handles the differential delay between the different backhaul links. In one example, the traffic processing unit performs frame selection at preset time intervals. If a frame for the call is late from one of the backhaul links, then that frame is not considered in the frame selection process. Other methods have been proposed to buffer the frames so that frame selection is performed on frames received over each of the backhaul links.
For forward call traffic, the base stations presently handle the differential delay between the different backhaul links. The traffic processing unit broadcasts frames for the call over each of the backhaul links to the base stations. The base stations then transmit the same frames in the sequence to the mobile according to a preset time interval. If one of the base stations receives frames in the sequence late (i.e., each frame is received after the preset time interval), then those frames are discarded. This is unfortunate as this base station may have the strongest signal with the mobile device over which to transmit the frames to the mobile device. Thus, it is presently a problem that there is differential delay in the backhaul links for forward call traffic.